This invention relates to water craft and amphibious vessels and more particularly to such vessels of the hydrofoil type.
In recent years hydrofoils and similar devices have become of increasing importance in the movement of vessels over the water. As is well known, hydrofoils commonly are in the form of thin generally planar structures which are suspended beneath the vessel and extend in directions substantially perpendicular to the vessel's direction of movement. The hydrofoils are usually maintained in rigid relationship to the hull of the vessel and are contoured such that at high speeds the vessel rises at least partially out of the water and rides or "planes" on the foils.
The various hydrofoil devices employed heretofore have been found to be deficient in several respects. For example, the operation of hydrofoil vessels was easily impaired by obstructions, such as floating logs and pilings, grasses, weeds, shallow water, rocks, sandbars, mudflats, and shores, and it was difficult to adapt the vessels for landing on and launching from beaches and in general for amphibious use.
Another problem resulted from the operation of the vessel in heavy seas or other conditions involving substantial wave action. The rigidly mounted hydrofoils previously employed produce rough rides or instability, and it was difficult to mount an appropriate suspension between the main portion of the vessel and the foils in an economical manner.
Still further difficulties in the operation of conventional hydrofoils resulted from excessive frictional drag due to the movement of their wetted surface area through the water, with the corresponding deleterious effect on the speed of the vessel. Attempts to reduce the wetted surface area have met with the problem that, at lower speeds, the reduced lifting effect did not adequately support the vessel. As a result the drag of the foils plus the drag of the hull was substantially more than the drag of the hull alone would have been.
In addition, there were difficulties in powering hydrofoil craft, because quite a long propeller shaft was required to keep the propeller under water when the vessel rode onto the foils. The long shaft with its attendant bearings and mounting brackets added frictional drag and contributed to the expense of constructing hydrofoil craft, with the result that they were slower, less economical of fuel, and more expensive than they otherwise would be.
Some attempts have been made to build vessels competitive with hydrofoil vessels which overcame some of the problems of hydrofoils by providing inclined rotating floats. These were not technically hydrofoil craft and did not cut through the water but behaved more like displacement hull craft with moving skins. They were limited by large wave-making drags, firstly because all displacement hulls tend to be so limited, and secondly because the submerged portion made a poor shape for a displacement hull, with the result that the shape of the "hull" formed by the portion of the float in the water necessitated the expenditure of considerable energy in pushing water sideways. In addition, rotating floats previously envisioned were severely limited by their failure to provide a sharp edge from which the water can leave the float, with the result that water was sucked up behind the floats, greatly decreasing the speed of these vessels, and actually pulling them down deeper into the water as their speed increased, an effect actually opposite to the more desirable lift provided by hydrofoil craft at speed.
A cure was sought for this in providing ridges which beat down the water when the floats were driven at a speed well in excess of that of the vessel. However, such an arrangement required too much power for optimal results, as any lifting action came primarily from the work of the engines in rotating the floats past the water, rather than from the forward motion of the vessel to support it in the manner of hydrofoils and hydroplanes. There was also the danger of sudden increases of resistance and strong downward suction in the case of power failure or power reduction when attempting to slow down from high speed, resulting in risk to craft and cargo. Furthermore, as the ridge type floats did not cut into the water in the manner of hydrofoils, they lacked purchase for rapid turns which hydrofoil craft with properly oriented foils display; and being unable to cut through waves and lacking suspension to allow the floats to rise over the waves, the water craft were buffeted at high speed with attendant instability, discomfort and danger.